This invention relates generally to gas turbine engines and more particularly to discrete airflow platforms disposed between adjacent fan blades in such engines.
A turbofan gas turbine engine used for powering an aircraft in flight typically includes, in serial flow communication, a fan assembly, a low pressure compressor or booster, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. The combustor generates combustion gases that are channeled in succession to the high pressure turbine where they are expanded to drive the high pressure turbine, and then to the low pressure turbine where they are further expanded to drive the low pressure turbine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan assembly and the booster via a second rotor shaft.
The fan assembly includes a plurality of circumferentially spaced apart fan blades extending radially outwardly from a rotor disk. The fan blades generally comprise an airfoil section and an integral dovetail root section. The dovetail section is slidably received in a complimentary configured dovetail slot formed in the rotor disk so as to attach the blade to the rotor disk. Fan blades are typically made of either a metal, such as titanium, or a composite material.
While known fan assemblies are highly effective under normal operating conditions, they can sometimes be susceptible to foreign object damage, i.e., damage resulting from the ingress of foreign objects such as birds or hailstones. If a large foreign object impacts a fan blade, the fan blade, or a portion thereof, could break off from the rotor disk. A detached fan blade could damage adjacent fan blades and create a large imbalance in the fan assembly. Furthermore, if not contained by the fan casing, a detached fan blade could cause considerable damage to the aircraft powered by the engine. To reduce the amount of blade material lost during an ingestion event, it is known to design fan assemblies so as to permit limited rotation of the blade root section within the dovetail slot in response to an extreme force exerted on the blade. This rotation will often prevent breakage of the blade at its base.
During engine operation, ambient airflow is channeled between the rotating blades and pressurized thereby for generating thrust for powering the aircraft in flight. A radially inner flowpath boundary for the airflow channeled between the blades is provided by fan platforms located between adjacent fan blades, near the rotor disk. It is known to produce fan blades having integrally formed platforms. However, this means that the centrifugal loads from both the fan blade airfoils and the platforms must be carried by the fan blade dovetails, which requires that the dovetails be suitably large, which in turn requires a suitably large rotor disk for accommodating all of the centrifugal loads within acceptable stress limits. Furthermore, damage to such an integrally formed platform will often require replacement of the entire blade. And it is impractical from a manufacturing standpoint to integrally form the platforms with the blade in the case of composite fan blades.
Accordingly, fan assemblies have been developed using discrete platforms independently joined to the rotor disk between adjacent fan blades. These separate platforms must have suitable strength for accommodating both centrifugal loads and impact loads, such as those due to a bird strike, during operation. One such configuration is a hybrid construction comprising a forged aluminum machined structural body having a composite flowpath surface bonded thereto. The structural body portion is formed with a straight wall box shape for ease of machining. Because the adjacent fan blades have a curved contour, this straight wall construction leads to hard body pinch points between the fan blade, fan platform and rotor disk during foreign objection ingestion events. Such pinch points limit the fan blade rotation capability in response to a blade being struck by a foreign object.
Furthermore, the composite flowpath surface must be secondarily bonded in an additional operation and requires the use of redundant bolted fastening features through the structural side walls to insure flowpath retention. And the aluminum structural body must be shotpeened and primed prior to composite bonding. This results in a relatively heavy platform that is expensive to manufacture.
Accordingly, there is a need for a lightweight, easily manufactured fan platform that does not limit the rotation capability of the adjacent fan blades.